Because of their high volumetric efficiency and thus their small size, multilayer ceramic capacitors (MLC's) are the most widely used form of ceramic capacitors. These capacitors are fabricated by stacking and cofiring thin sheets of ceramic dielectric on which an appropriate electrode pattern is printed. Each patterned layer is offset from the adjoining layers in such manner that the electrode layers are exposed alternately at each end of the assemblage. The exposed edges of the electrode pattern are coated with a conductive material which electrically connects all the layers of the structure, thus forming a group of parallel connected capacitors within the laminated structure. Capacitors of this type are frequently referred to as monolithic capacitors.
The thin sheets of ceramic dielectric used for the fabrication of MLC's are commonly referred to as "green tapes" and are comprised of a thin layer of finely divided dielectric particles which are bound together by an organic polymeric material. Unfired green tapes are prepared by slip casting a slurry of the dielectric particles dispersed in a solution of polymer, plasticizer and onto a carrier such as polypropylene, Mylar.RTM. polyester film or stainless steel and then adjusting the thickness of the cast film by passing the cast slurry under a doctor blade.
Metallizations useful in producing conductors for multilayer capacitors normally comprise finely divided metal particles applied to green tape in the form of a dispersion of such particles in an inert liquid vehicle. Although the above-described "green tape" process is more widely used, there are nevertheless other procedures with which dielectric compositions of the invention can be used to make MLC's. One technique is the so called "wet process". In one aspect, this may involve passing a flat substrate through a falling sheet of dielectric slip one or more times to build up a dielectric layer (see Hurley et al., U.S. Pat. No. 3,717,487). Another way of carrying out the "wet process" is to brush a number of thin layers of dielectric slip onto a substrate to build up a thick dielectric layer (see Burn. U.S. Pat. No. 4,283,753).
Another method of making MLC's involves forming a paste of the dielectric material and then alternately screen printing the dielectric and metal layers with intervening drying steps until the designed structure is complete. A second electrode layer is then printed atop the dielectric layer(s) and the entire assemblage is cofired.
Monolithic multilayer capacitors are typically manufactured by cofiring barium titanate based formulations and conductive electrode materials in oxidizing atmospheres at temperatures of 1200.degree.-1400.degree. C. This process yields durable, well sintered capacitors with high dielectric constant, e.g., greater than 1000. However, firing under these conditions requires an electrode material with high melting point, good oxidation resistance at elevated temperatures, sinterability at the maturing temperature of the dielectric, and minimal tendency to interact with the dielectric at the sintering temperature. These requirements normally limit the choice of electrode materials to the noble metals platinum and palladium, or to alloys of platinum, palladium and gold. See also U.S. Pat. No. 3,872,360 to J. L. Sheard which is directed to the preparation of monolithic multilayer capacitors.
Significant savings in electrode costs could be realized if dielectric materials could be modified to (1) yield good dielectric properties (high dielectric constant and low dissipation factor) after firing in reducing atmospheres so that base metals could be used as electrodes and/or (2) sinter at temperatures of 950.degree. C. or lower so that silver, which is significantly less costly than the other noble metals but has a lower melting point (962.degree. C.), could be used in electrode formation.
Attempts have been made to modify barium titanate ceramics so that they may be fired in reducing (e.g. hydrogen) or inert (e.g. argon, nitrogen) atmospheres. The use of this approach has been somewhat limited in that the electrical properties, e.g., dielectric constant, dissipation factor, temperature coefficient of capacitance, etc., are compromised as compared with those of conventional air-fired compositions. In addition, maintaining an inert or reducing atmosphere involves an additional production cost as compared to firing in air. Exemplary of this approach is Buehler, U.S. Pat. No. 3,757,177, disclosing capacitors of base metal electrodes (e.g., Ni, Co, Fe) and modified barium titanate (MnO.sub.2, Fe.sub.2 O.sub.3, CoO.sub.2, CaZrO.sub.3) fired in an inert atmosphere about 1300.degree. C. (col. 3, lines 33-34).
Several attempts have been made to reduce the maturing temperature of dielectrics by mixing high temperature ferroelectric phases (titanates, zirconates, etc.) with glasses which mature at relatively low temperatures. Examples of this approach are given in Maher U.S. Pat. No. 3,619,220, Burn U.S. Pat. No. 3,638,084. Maher U.S. Pat. No. 3,682,766, and Maher U.S. Pat. No. 3,811,937. The drawback of this technique is that the dilution effect of the glass often causes the dielectric constant of the mixture to be relatively low.
Another technique for lowering the sintering temperature of titanate-based dielectrics is by the use of "sintering aids". Additions of bismuth oxide or bentonite to barium titanate lowers the maturing temperature to about 1200.degree. C. (Nelson et al. U.S. Pat. No. 2,908,579). Maturing temperatures of 1200.degree.-1290.degree. C. may be attained by addition of phosphates to titanates as described in Thurnauer et al. U.S. Pat. No. 2,626,220. However, in each of these cases, the decrease in maturing temperature is not sufficient to permit the use of cofired silver electrodes, and dielectric properties are often degraded.
There existed a need for a composition which would produce a high dielectric constant (e.g., 1000 or above) and low dissipation factor (e.g., less than 5%, preferably less than 2%) and sinters in air at low temperatures (e.g., less than 1000.degree. C. or less). This would permit cofiring with silver or palladium/silver electrodes and hence would greatly reduce the cost of high dielectric constant multilayer capacitors.
N. N. Krainik et al. (Soviet Physics-Solid State 2, 63-65, 1960) report solid solutions between, inter alia, PbTiO.sub.3 and PbMg.sub.0.5 W.sub.0.5 O.sub.3. Apparently a wide range of compositions with 0-80% PbTiO.sub.3 was investigated (see FIG. 2). No suggestion was made as to the manufacture of multilayer capacitors. In a second article from the same laboratory, G. A. Smolenskii et al. (Soviet Physics-Solid State 3, 714, 1961) report investigating certain solid solutions, including those of Krainik et al. Firing was similarly done in a PbO vapor atmosphere. Phase transitions are discussed. In what is apparently a third article in this series, A. I. Zaslavskii et al. (Soviet Physics-Crystallography 7, 577, 1963), X-ray structural studies are reported.
Brixner U.S. Pat. No. 3,472,777 discloses the manufacture of ferroelectric ceramic disks by a two-step firing process. Each firing step is taught to occur in the range 800.degree.-1200.degree. C. in air. In the sole example, firing was at 1050.degree. C. Brixner discloses various dielectric compositions such as PbMg.sub.1/3 Ti.sub.1/3 W.sub.1/3 O.sub.3 and Y-containing compositions.
More recently, Bouchard has very successfully approached the problem of dielectric compositions having low firing temperatures and dielectric constants as high as 6000 for use in Z5U-type capacitors. These substituted lead titanate compositions correspond to the following formula:
______________________________________ (Sr.sub.x Pb.sub.1-x TiO.sub.3).sub.a (PbMg.sub.r W.sub.s O.sub.3).sub.b, wherein x = 0-0.3 r = 0.45-0.55 a = 0.35-0.5 s = 0.55-0.45 and b = 0.5-0.65 .SIGMA.(r + s) = 1, and .SIGMA.(a + b) = 1. ______________________________________
Such materials are disclosed in U.S. Pat. Nos. 4,048,546, 4,063,341, and 4,228,482, all to Bouchard. More recently, U.K. Patent Application No. 2,115,400A discloses quite similar compositions which have low sintering temperatures corresponding to the formula: EQU PbTi.sub.1-x-y Mg.sub.x W.sub.y O.sub.3,
in which x and y range from 0.25 to 0.35. These materials are made by mixing the corresponding metal oxides and calcining the mixture at 700.degree.-750.degree. C. The materials are sintered at 800.degree.-950.degree. C. which is below the melting point of silver. Some of the compositions of the U.K. application are identical in composition to those of Bouchard and are therefore expected to have the same properties.
Notwithstanding the substantial progress in attaining higher dielectric constants, the electronics industry foresees the need for dielectric compositions having still higher dielectric constants (K) on the order of 8000 and even higher, which nevertheless can still used with conventional silver-containing electrodes such as 85/15 and 70/30 palladium/silver electrodes.